Recently, with the progress of information communication technique or the prevalence of various electric apparatus, the influence of unnecessary electromagnetic noises exerted onto precision apparatus associated devices has raised problems. For the measurement of electromagnetic noises, a radio wave dark room (anechoic room) where there is no reflection of electromagnetic waves is used, and a radio wave absorbent is used in the inner wall of the radio wave dark room. Moreover, in order to prevent reception trouble from being caused by reflection of television waves by high-rise buildings or the like, the radio wave absorbent is used in the outer wall of the building or the like.
As a conventional radio wave absorbent, for example, used is a radio wave absorbent having a characteristic that a reflection attenuation in a frequency band of 90 MHz to 350 MHz is 20 dB or more. As the radio wave absorbent, for example, a radio wave absorbent obtained by sintering a nickel-zinc system ferrite material (Japanese Patent Application Laid-open Nos. 200303/1991, 129123/1993, 243023/1993, 84622/1994, and the like) and a radio wave absorbent obtained by sintering a magnesium-zinc system ferrite material (Japanese Patent Application Laid-open Nos. 72925/1989, 301524/1989, and the like) are exemplified.
However, the conventional radio wave absorbent which is obtained by sintering the nickel-zinc system ferrite material has a problem that a raw-material nickel is expensive.
On the other hand, the magnesium-zinc system ferrite material has a sintering temperature of around 1250 to 1300.degree. C., which is higher than the sintering temperature of about 1100.degree. C. of the nickel-zinc system ferrite material. Therefore, when the radio wave absorbent is industrially manufactured by sintering the magnesium-zinc system ferrite material, there is a problem that a high-temperature sintering furnace is necessary separately from a sintering furnace for the nickel-zinc system ferrite material.
Furthermore, the matching thickness in which all incoming radio waves are absorbed without causing reflection is important in the radio wave absorbent, but the radio wave absorbent obtained by sintering the conventional magnesium-zinc system ferrite material has a matching thickness of 8 mm or more, and is thicker than the radio wave absorbent obtained by sintering the nickel-zinc system ferrite material (matching thickness of 7 mm or less). The difference of about 1 mm in matching thickness exerts a remarkable influence on the total weight of the radio wave absorbent for use in the inner wall of the radio wave dark room or the outer wall of the building or the like, and the reduction of the matching thickness is constantly requested for in the radio wave absorbent.
The present invention has been developed in consideration of the circumstances described above, and an object thereof is to provide a radio wave absorbent which has a matching thickness of less than 8 mm and which can be obtained by sintering the material at a relatively low sintering temperature.